Purification of thiophenols by treatment with aluminum thiophenoxides



5 Claims. 01. 260-609) This invention relates to the art' of separatingthionited States PatcntfO Ice phenols from tar acids. More'particul'arlyit mama;

purifying thiophenols contaminated with tar acids by selective reaction.of the tar acids with aluminum thiophenoxides to form aluminumphenoxides.

By tar acids I refer to those constituents present in lcoal-tardistillates, certain cracked petroleum distillates .and the like, oftenreferred to collectively phenols, which are soluble in dilute causticsoda, giving sodium cresylate. I particularly refer to a mixture of thesimpler monohydric phenols boiling below about 230 C.

and consisting almost entirely of a mixture of phenol, 7

.methylphenols (cresols) and. dimethylphenols (xylenols),

with lesser amounts occasionally present of ethylphenols landtrimethylphenols.

At present, thiophenols are principally obtained from ftwo sources: as aby-product from the caustic extraction of petroleum distillates and bysynthetic methods starting with benzene. In one widely used directsynthesis tech- ;nique, the benzene is converted to benzene sulfonylchloride by treatment with a molar excess of chlorosulfonic acid. Theproduct is then converted to thiophenol by reduction in the presence ofa metal-acid system. While :the resulting product is of high purity, andparticularly useful for paint, dyestuffs, and pharmaceuticalapplicaztions, it is relatively expensive. This prevents its wide-:spread use for many applications.

In obtaining thiophenols from petroleum distillates rersulting fromoil-cracking processes, the tar acids and .thiophenols are recovered byextracting the petroleum distillate with aqueous caustic solution toproduce Waterrsoluble tar acid salts. In this process of extraction,thiophenols present are also removed by the aqueous caustic :solutioninasmuch as the thiophenols are even stronger :acids than the phenols ortar acids themselves, The quantity of thiophenols in the original sourcematerial varies widely, being sometimes as little as one percent by:weight of the phenols and ranging as high as 25 percent and above. Thethiophenols consist principally of thiophenol itself and mixedthiocresols and thioxylenols.

In US. Patent 2,767,220, a process is set forth for purifyingthiophenol-contaminated tar acids that are substantially free of'neutralhydrocarbon oils. In this process, the feedstock, consisting principallyof tar acids and thiophenols, is contacted with aqueous methanol andwith a low boiling parafiinic naphtha fraction in a continuouscountercurrent extraction zone. The aqueous methanol; solution dissolvessubstantially all the tar acids, and the naphtha fraction dissolves thethiophenols. The naphtha fraction is distilled off to yield thethiophenols' as still bottoms. Inasmuch as the purification'of the taracids is the desideratum of this process, the thiophenols recoveredfromthe para'fiinic naphtha fraction invariably contain from 2 to 20 percenttar acids by weight. Complete removal of the tar acids fro'mthethiophenols does not take place even when the thiophenols arefractionally-distilled in a highly eflicient column, such as a50-plate-packed; tower. Thiophenol from such a distillation containsaphours.

2,954,403 Patented Sept. 27, 1960 proximately 1.5 percent phenol.Efficient fractional distill'ation is likewise unsuccessful in eifectinga removal of the tar acids from the mixed thiocresols and thioxylenols.These thiophenols contain even higher amounts of closeboilin'gj taracids. 2

The process of the present invention provides a.- thin-' phenol ofimproved purity with respect to tar acid con;

tent. In accordance with this invention, a mixture of 'thiophenols andtar acids is treated with a preformed aluminum salt of a thiophenol. Arapid and irreversible interchange of the aluminum thiophenoxide withthe tar acids then occurs whereby the tar acids selectively formaluminum phenoxides. The thiophenols are then separated from therelatively. nonvolatile aluminum phenoxides, preferably 'bydistillation. A preferred aluminum thiophenoxide reactant for use in thepresent invention is the aluminum salt of the thiophenol that ispresent. Thereby the only thiophenolic by-product of the interchange isthe same as the thiophenol being purified.

It has been found that this method is suitable for rendering thiophenolscontaining tar acids in amounts upto 50' percent by weight of themixture substantially free from tar acid contamination. The method isparticularly preferred where the thiophenol has a tar acid content of upto20 percent by weight of the mixture. In general, where thiophenolitself is to be purified, the contaminant present is usually phenol. Itis believed that an azeotrope is formed thereby preventing theseparation of the thiophenol and phenol by conventional distillationtechniques. Mixed thiocresols are usually contaminated with o-cresol.The thioxylenols generally contain a contaminating amount ofclose-boiling cresols and xylenols. This process is particularly usefulfor puritying tar acid-contaminated thiophenol, thiocresols, andthioxylenols.

In my copending application Serial No. 747,589, filed July 10, 1958, aredescribed various techniques for preparing aluminum thiophenoxide salts.In general, a preferred method involves preparing an aluminum amalgam,preformed or formed in situ. In a preferred method for preforming anactive aluminum amalgam,

granular aluminum (20-30 mesh) is cont-acted'with a saturated etherealsolution of mercuric chloride for about 0.5 minute at the boiling pointof the solution. The

' grams of an aluminum amalgam prepared as above were rapidlytransferred to a 5-liter flask fitted with a stirrer, a thermowell, andan efficient reflux condenser. Thiophenol (98 percent assay), 2960grams, was added, and the apparatus was purged with nitrogen. Reactioncommenced at 100 C. and became extremely vigorous at 121 C. Thetemperature rapidly rose to C. even though theheating mantle wasremoved. Gas evolution ceased after 20 minutes.

A total of 2131 grams of thiophenol was then recovered by distillationon aste'am bath, with stirring, at 15 mm. The residual was cooled andtreated with 500 ml. of hexane. Stirring was continued for one hour, andthe slurry was rapidly filtered. The cake Was washed with a small amountof hexane and then was transferred to a vacuum desiccator for dryingat 5mm. for several Undue contact with air was avoided'throughout theprocess. The dried cake, a grayish-white powder,

aluminum phenoxide.

such as mercuric chloride or cyanide, together with the thiophenol whosealuminum salt is to be prepared. A temperature between 30 and 175 C. ispreferred for the reaction betweenthe aluminum and the thiophenol. Areaction product of aluminum and a thiophenol may also be prepared byreacting aluminum directly with a thiophenol in the absence of a mercurycompound. However, to accomplish this, reaction at superatmosphericpressure at a temperature between 200 and 400 C. is required.

Upon addition of the aluminum thiophenoxide salt to the mixture ofthiophenol and tar acid, preferably in an amount sufficient to provideat least /3 gram atom of aluminum per gram mole of phenol present in thethiophenol, an almost immediate exothermic reaction occurs. If moistureis present, additional amounts of aluminum thiophenoxide are required.However, an excess of aluminum thiophenoxide above the stoichiometricamount is ordinarily not required for substantially complete removal ofthe tar acids if moisture is not present.

If addition of aluminum thiophenoxide per se, for

' example, is made at room temperature to a mixture of thiophenol andphenol, the solid aluminum thiophenoxide almost immediately begins todissolve in the mixture of thiophenol and phenol. Although aluminumthiophenoxide is not very soluble in cold thiophenol, a suflicientamount dissolves to react with phenols present to form Since thisreaction is exothermic, the evolution of heat further promotes solutionof the thiophenoxide. This reaction is therefore seen as capable ofoccurring at a temperature range of from to 200 C., particularly becauseof its exothermic nature. The

' greater the proportion of phenols present, the more rapid is thereaction. Also, the reaction is initiated at a lower temperature andproceeds more vigorously than when a lower concentration of phenol ispresent. The reaction is ordinarily complete within several seconds toseveral minutes, depending upon the initial temperature employed. In thecourse of the reaction, no evolution of hydrogen occurs, but aninterchange of aluminum occurs so that upon use of a stoichiometricamount of aluminum thiophenoxide, all the tar acids present are combinedwith the aluminum.

This procedure offers the advantage that temperatures 0 above those usedfor vacuum distillation are not required. Further, no hydrogen evolutionoccurs in situ during formation of the aluminum phenoxide. Also the onlyby-product of the interchange is a thiophenol. By

a thiophenol in the presence of an inorganic mercury salt.

Where thesubsequent presence of the mercury may be undesirable, thealuminum thiophenoxide may still be prepared in the presence of themercury salt, and most of the formed mercury may then be readily removedby simple mechanical separation. traces of mercury, the thiophenoxidemay be dissolved in thiophenol, and the mercury separated therefrom byfiltration or high-speed centrifugation. The thiophenol is then removedby distillation. Or the aluminum thiophenoxide containing traces of.mercury may be used as such, the mercury being subsequently removed fromdistillate fractions of the purified thiophenol.

The following examples illustrate this invention but are not intended aslimitations thereof:

Example 1 Reaction of aluminum thiocresoxide with thiocresolphenolmixture.-An amount of 465 grams of mixed thiocresols free from tar acidsand 1.80 grams of mercuric chloride were heated to 165 C. Fifteen gramsof granular aluminum and an additional 0.40 gram of mercuric chloridewere added in portions to maintain a vigorous 89 percent.

the excess thiocresols. removed by distillation under vacuum. To amixture reaction. The reaction was complete after one hour.Approximately 208 grams of the thiocresols originally present combinedwith the added aluminum to form aluminum thiocresoxide.

The reaction mixture was then cooled to 105 C., and 126 grams of phenolwas added. The temperature rose sharply to 129 C. because of theexothermic nature of thereaction that occurred. The resulting dark,clear solution was maintained at 100 C. for one hour; the pressure wasthen gradually reduced until 405 grams of distillate was collected. Thedistillate consisted of 100 percent thiocresol as determined byvapor-phase chromatography. No phenol was detectable. The latter,however, was readily recovered from the residual aluminum salts byhydrolysis.

Example 2 Conversion of aluminum thiocresoxide to aluminum0-cres0xide.--A mixture of 188 grams (0.474 mole) of aluminumthiocresoxide, prepared by the reaction of aluminum and mixedthiocresols, and 208 grams (1.92 moles) of o-cresol was distilled to apot temperature of 200 C. at 5 mm. Hg. The overhead product contained156 grams (1.26 moles) of thiocresols and 66 grams (0.612 mole) ofo-cresol. The nonvolatile residue weighed 172 grams. Thus the conversionof aluminum thiocresoxide to an aluminum salt of o-cresol was at leastExample 3 Exothermic and irreversible nature of aluminum thiophenoxideinterchdnge.-Excess tar acid-free mixed thiocresols, 465 grams (3.74moles), were reacted with 15.00 grams (0.556 gram-atom) of aluminum inthe presence of mercuric chloride. On completion of the reaction, thetemperature Was lowered to 105 C. at which point the aluminumthiocresoxide formed a light gray slurry in The unreacted thiocresolswere consisting of 248 grams (2.00 moles) of dry mixed thiocresolisomers and 126 grams (1.34 moles) of dry phenol was then added thepreviously prepared aluminum thiocresoxide. A dark, clear solutionformed instantly and the temperature rose sharply to 129 C., thusindicating a significantly exothermic reaction.

(In a separate experiment, a similar temperature rise occurred on theaddition of o-cresol to aluminum thiophenoxide and thiophenol.) Thereaction mixture was maintained at 100 C. for one hour. Finally, 409grams (3.29 moles) of pure thiocresols Was recovered by a Vigreauxdistillation of the reaction mixture to a pot-temperature of 105 C. at10 mm. Hg. All of the phenol remained in the pot residue despite itslower boiling point (105 C./50 mm.-Hg) as compared to that of thethiocresols (108 C./50 mm. Hg). The result is interpreted to indicatethat the interchange of phenol with aluminum thiocresoxide is complete,and, under the distillation conditions, essentially irreversible.

To remove remaining.

The mechanism that occurs in the reactions of this invention isconsidered a'highly complex one. While I do not desire to have the scopeof this invention re- I stricted by any explanation profiered, it isconsidered num phenoxides. This interchange occurs somewhat surprisinglydespite the fact that thiophenol is a considerably stronger acid thanphenol, and hence thiophenol might ordinarily be expected to be morereactive than phenol. However, as shown herein, once this intertospecific preferred embodiments, I do not desire to be limited by theillustrative examples given or the specula tive mechanisms postulatedfor this reaction. The scope of this invention accordingly should bedetermined in accordance with the principles and claims thereof.

I claim:

1. The process for purifying a tar acid-contaminated thiophenol whichcomprises reacting a mixture of a thiophenol and a tar acid with analuminum thiophenoxide to react selectively with the tar acid to form analuminum salt thereof and a thiophenol of said thiophenoxide, andseparating the thiophenol from said salt.

2. The process for recovering a thiophenol in substantially pure formfrom a mixture containing a thiophenol and a tar acid which comprisesadding to said mixture under reactant conditions an amount of analuminum thiophenoxide suflicient to provide at least /3 gram atom ofaluminum per mole of tar acid present to react selectively with the taracid to form an aluminum salt thereof and a thiophenol of saidthiophenoxide, and distilling off the thiophenol substantially free fromtar acid.

3. The process according to claim 2 wherein said 6 aluminumthiophenoxide is the aluminum salt of a thiophenol present in saidmixture.

4. The process for recovering a thiophenol in substantially pure formfrom a mixture containing a major portion of a thiophenol and a minorportion of a tar acid which comprises reacting an amount of an aluminumthiophenoxide sufiicient to provide at least /3 gram atom of aluminumper mole of tar acid present with said mixture at a temperature between0 and 200 C. at which a reaction occurs and an aluminum phenoxide and athiophenol are formed, and distilling off the thiophenol from themixture in substantially pure form.

5. The process for recovering a thiocresol in substantially pure formfrom a mixture containing a major portion of a thiocresol and a minorportion of cresol which comprises adding to said mixture under reactantconditions an amount of aluminum thiocresoxide suflicient to provide atleast A gram atom of aluminum per mole of cresol present to reactselectively with the cresol to form aluminum cresoxide and thiocresol,and distilling off the thiocresol substantially free from cresol.

No references cited.

1. THE PROCESS FOR PURIFYING A TAR ACID-CONTAMINATED THIOPHENOL WHICHCOMPRISES REACTING A MIXTURE OF A THIOPHENOL AND A TAR ACID WITH ANALUMINUM THIOPHENOXIDE TO REACT SELECTIVELY WITH THE TAR ACID TO FORM ANALUMINUM SALT THEREOF AND A THIOPHENOL OF SAID THIOPHENOXIDE, ANDSEPARATING THE THIOPHENOL FROM SAID SALT.